NEW YORK (CNN) --Power began to flicker on late Thursday evening, hours after a major power outage struck simultaneously across dozens of cities in the eastern United States and Canada.

By 11 p.m. in New Jersey, power had been restored to all but 250,000 of the nearly 1 million customers who had been in the dark since just after 4 p.m., a spokeswoman for Public Service Energy and Gas said.

Power was being restored in Pennsylvania and Ohio, too.

In New York City, however, Con Edison backed off previous predictions that power for most of the metropolitan area would be restored by 1 a.m. Friday. The power company had predicted that residents closer to Niagara Falls in upstate New York would have to wait until 8 a.m.

The outage occurred quickly and rippled across a large area. Cities affected included New York, Cleveland, Ohio, Detroit, Michigan, and Toronto and Ottawa, Canada.

In just three minutes, starting at 4:10 p.m., 21 power plants shut down, according to Genscape, a company that monitors the output of power plants.

It was unclear what caused the outage, although state and federal officials agreed that it was not terrorism.

One possibility was a lightning strike in the Niagara region on the U.S. side of the border, according to the Canadian Department of National Defense. A spokeswoman for the Niagara-Mohawk power grid said the cause was still unknown, but that it was not a lightning strike.

A spokesman for the Canadian prime minister's office said the cause was a fire at a Con Edison power plant in New York.

Canadian Defense Minister John McCallum blamed an outage at a nuclear power plant in Pennsylvania, but the state's Emergency Management Agency said there had been no problems at any of the state's five nuclear plants and that all were operating normally.

The outage stopped trains, elevators and the normal flow of traffic and life. In Michigan, water supplies were affected because water is distributed through electric pumps, a governor's spokeswoman said.

By 6 p.m. the power was being restored in parts of the affected area, starting with the northern and western edges, New York Mayor Michael Bloomberg said at a news conference.

At 9:30 p.m. the Long Island Power Authority said its 1.2 million customers were beginning to see power restored, although it could take hours to get everyone back on line in the New York area.

New York subways resumed limited service around 8 p.m., according to the Port Authority of New York and New Jersey. It took 2.5 hours to evacuate passengers from stalled subway trains, the Metropolitan Transportation Authority said. (Full story)

Airports across the affected region experienced delays and some shut down temporarily. By early evening, two New York area airports and the Cleveland airport were fully operational, although continued delays should be expected. Planes were still grounded at New York's JFK Airport as of 8:30 p.m.

The airports were operating on backup power, officials said. "Expect extended flight delays and long wait times," a United Airlines spokesman said.

The New York Stock Exchange announced plans to open on schedule Friday, using emergency power if necessary.

Bloomberg mobilized 40,000 police officers and the entire fire department overnight to maintain order. As of late afternoon, no reports of looting or other disturbances had been reported.

New York Gov. George Pataki declared a state of emergency for the state and deployed additional state police.

Bryan Lee, a spokesman for the Federal Energy Regulatory Commission, said it appeared that a "cascading blackout" destabilized the Niagara-Mohawk power grid as far north as Canada and as far west as Detroit and Cleveland.

Crowds line up for buses at the Port Authority bus terminal in New York.

Government officials said quickly ruled out the "Blaster" computer worm as a cause. The worm was spreading from computer to computer Wednesday and was initially considered a potential cause.

The outage did slow the Internet, however, because Web sites powered from servers in affected cities were unable to respond to requests to view the pages. Also, experts said, the Internet may be trying to reroute itself to cover for unresponsive servers.

The 21 plants went off line because when the grid is down there is no place for the power output to go. Unlike fuels such as coal or natural gas, electricity is difficult to store. Power is generated as it is used.

At least 1.5 million people in central and northern New Jersey were without power late Thursday and there was no train service for homebound commuters.

Amtrak stopped all trains leaving the New York city area, said spokesman Marc Magliari.

The north-south trains were not running north of Philadelphia, Magliari said. But the Empire line in New York state, the east-west lines to Harrisburg-Lancaster in Pennsylvania, and the Washington, D.C.-Chicago runs were all operating normally.

Amtrak operations also were down in Michigan between Detroit and Dearborn, Michigan, and Pontiac, Michigan.

Emergency officials in New York City were trying to get commuters across the Hudson River by ferry. Traffic signals were inoperable, and emergency officials urged motorists to stay off the roads.

The New York City Police Department said a number of people were trapped in elevators for awhile. Thousands of people left buildings and walked into the streets. (New Yorkers stay calm)

"We are going to have a situation where people are going to have to walk a long distance. They need to be careful," Bloomberg said. "Our advice is to go home, open up your windows, drink a lot of liquids."

Cars sit stopped about three-quarters of the way up the first hill of the Magnum XL200 ride at Cedar Point Amusement Park in Sandusky, Ohio.

President Bush was in California for a fund-raising dinner but addressed the outage in a short speech for reporters.

"I want to thank the people for their calm response to this emergency situation," he said. "It's been remarkable to watch on TV how resolved the people have been in dealing with this situation. I know their neighbors are thankful for the proper and calm response."

Bush said communication between local, state and federal officials was "quick and thorough."

"We're better organized today than we were two and a half years ago to deal with an emergency and the system responded well," he said.

The last big blackout in the United States took place almost exactly seven years ago, August 11, 1996, when some 4 million customers in nine Western states and parts of Mexico lost power for as long as 10 hours. (Other blackouts)

In 1977, a blackout left some 9 million people in New York City without power for up to 25 hours starting on July 13.

In the Great Northeast Blackout of 1965, the largest in U.S. history, at least 25 million people in New York, New England and portions of Pennsylvania and New Jersey lost electricity for a day starting late in the afternoon of November 9.